How much similarity can be seen between the current body plans of How do you know if two related organisms are members of the same species? This is the answer key's answer so if someone could actually go on the website that I will have down below or either paraphrase this There is considerable similarity in the body plans of ancient animals and today’s
animals. The basic body plan of the elephant was evident in the tiny pikaia, a
small swimming animal. The body plan of this animal was passed on to fish,
amphibians, reptiles, birds and mammals.

That would be great!